Academic literature on the topic 'Electronic Structure - Ultra-thin Epitaxial Films'

A combination of atom column-by-column scanning transmission electron microscopy and density functional theory shows how epitaxial strain alters the local electronic structure in mixed phase bismuth ferrite thin films.

Ni, Pd and Cr thin films were grown on (100)SrTiO3 surfaces by molecular beam epitaxy at substrate temperatures of TNJ, pd=650°C and Tcr =150°C. Electron energy-loss spectroscopy (EELS) and high resolution transmission electron microscopy (HRTEM) were applied to investigate the local electronic structure and the atomic structure of the interfaces, respectively. Analytical microscopy was carried out with a parallel energy-loss spectrometer (PEELS766) attached to a dedicated scanning transmission electron microscope (STEM) operated at 100keV, which has a point resolution of 0.22 nm. HRTEM studies were performed on a Jeol JEM ARM 1250 operated at 1250keV (0.12 nm point resolution). Conventional TEM and HRTEM experiments showed epitaxial orientation relationships between the thin metal films and the substrate for each interface.The electronic structure of the interfaces in terms of the site- and symmetry projected density of states (PDOS) above the Fermi-level can be extracted from the electron energy-loss near-edge structures (ELNES).

Des films minces d’oxydes de cuivre (Cu2O, Cu4O3 et CuO) ont été déposés à température ambiante sur des substrats en verre et en silicium par pulvérisation magnétron réactive. Une attention particulière a été portée à l’influence des conditions de synthèse (débit d’oxygène et pression totale) sur la structure et l’orientation préférentielle des dépôts. La pression totale est le paramètre principal influençant la texture des films de Cu2O et de Cu4O3. En revanche l’orientation préférentielle des films de CuO est contrôlée par le débit d’oxygène. Pour des films de Cu2O et de Cu4O3, un phénomène de croissance épitaxique locale (CEL) a été mis en évidence. Il résulte de l’utilisation d’une première couche qui joue le rôle d’une couche de germination lors du processus de croissance. Ainsi, les films peuvent croître avec une texture donnée indépendamment de leurs conditions de synthèse. Cet effet de CEL a été mis à profit pour élaborer des films biphasés (Cu2O + Cu4O3) qui présentent une microstructure originale. L’augmentation de la transmittance optique et du gap optique de films de Cu2O a été rendue possible par des traitements thermiques dans l’air qui permettent de diminuer la densité de défauts dans les films. Finalement, les propriétés optiques et la structure électronique des oxydes de cuivre qui ont été calculées par la méthode GW sont en accord avec des résultats expérimentaux obtenus par absorption optique, photoémission et spectroscopie de perte d’énergie des électrons
Copper oxide (Cu2O, Cu4O3 and CuO) thin films have been deposited on unmatched substrates by sputtering at room temperature. The influence of oxygen flow rate and total pressure on the film structure and preferred orientation has been studied. The total pressure is a relevant parameter to control the texture of Cu2O and Cu4O3 films, while the oxygen flow rate is effective to tune the preferred orientation of CuO films. Local epitaxial growth, where epitaxial relationship exists in columns of sputtered films, has been observed in Cu2O and Cu4O3 films by using a seed layer. The seed layer will govern the growth orientation of top layer via the local epitaxy, independently of the deposition conditions of top layer. Unusual microstructure that both phases have the vertically aligned columnar growth has been evidenced in biphase Cu2O and Cu4O3, which may relate to the local epitaxial growth of Cu2O. The lower resistivity than that in single phase films has been observed in this biphase film. Annealing in air can increase the transmittance of Cu2O films in visible region by the reduction of the impurity scattering, while the optical band gap is enlarged due to the partial removal of defect band tail. The optical properties and electronic structure of copper oxides calculated by GW approach with an empirical on-site potential for Cu d orbital, are in good accordance with experimental results from optical absorption, photoemission and electron energy loss spectroscopies

The term "topological insulator" (TI) represents a novel class of compounds which are insulating in the bulk, but simultaneously and unavoidably have a metallic surface. The reason for this is the non-trivial band topology, arising from particular band inversions and the spin-orbit interaction, of the bulk. These topologically protected metallic surface states are characterized by massless Dirac dispersion and locked helical spin polarization, leading to forbidden back-scattering with robustness against disorder. Based on the extraordinary features of the topological insulators an abundance of new phenomena and many exciting experiments have been proposed by theoreticians, but still await their experimental verification, not to mention their implementation into applications, e.g. the creation of Majorana fermions, advanced spintronics, or the realization of quantum computers. In this perspective, the 3D TIs Bi2Te3 and Bi2Se3 gained a lot of interest due to their relatively simple electronic band structure, having only a single Dirac cone at the surface. Furthermore, they exhibit an appreciable bulk band gap of up to ~ 0.3 eV, making room temperature applications feasible. Yet, the execution of these proposals remains an enormous experimental challenge. The main obstacle, which thus far hampered the electrical characterization of topological surface states via transport experiments, is the residual extrinsic conductivity arising from the presence of defects and impurities in their bulk, as well as the contamination of the surface due to exposure to air. This thesis is part of the actual effort in improving sample quality to achieve bulk-insulating Bi2Te3 films and study of their electrical properties under controlled conditions. Furthermore, appropriate capping materials preserving the electronic features under ambient atmosphere shall be identified to facilitate more sophisticated ex-situ experiments. Bi2Te3 thin films were fabricated by molecular beam epitaxy (MBE). It could be shown that, by optimizing the growth conditions, it is indeed possible to obtain consistently bulk-insulating and single-domain TI films. Hereby, the key factor is to supply the elements with a Te/Bi ratio of ~8, while achieving a full distillation of the Te, and the usage of substrates with negligible lattice mismatch. The optimal MBE conditions for Bi2Te3 were found in a two-step growth procedure at substrate temperatures of 220°C and 250°C, respectively, and a Bi flux rate of 1 Å/min. Subsequently, the structural characterization by high- and low-energy electron diffraction, photoelectron spectroscopy, and, in particular, the temperature-dependent conductivity measurements were entirely done inside the same ultra-high vacuum (UHV) system, ensuring a reliable record of the intrinsic properties of the topological surface states. Bi2Te3 films with thicknesses ranging from 10 to 50 quintuple layers (QL; 1QL~1 nm) were fabricated to examine, whether the conductivity is solely arising from the surface states. Angle resolved photoemission spectroscopy (ARPES) demonstrates that the chemical potential for all these samples is located well within the bulk band gap, and is only intersected by the topological surface states, displaying the characteristic linear dispersion. A metallic-like temperature dependency of the sheet resistance is observed from the in-situ transport experiments. Upon going from 10 to 50QL the sheet resistance displays a variation by a factor 1.3 at 14K and of 1.5 at room temperature, evidencing that the conductivity is indeed dominated by the surface. Low charge carrier concentrations in the range of 2–4*10^12 cm^−2 with high mobility values up to 4600 cm2/Vs could be achieved. Furthermore, the degradation effect of air exposure on the conductance of the Bi2Te3 films was quantified, emphasizing the necessity to protect the surface from ambient conditions. Since the films behave inert to pure oxygen, water/moisture is the most probable source of degeneration. Moreover, epitaxially grown elemental tellurium was identified as a suitable capping material preserving the properties of the intrinsically insulating Bi2Te3 films and protecting from alterations during air exposure, facilitating well-defined and reliable ex-situ experiments. These findings serve as an ideal platform for further investigations and open the way to prepare devices that can exploit the intrinsic features of the topological surface states
Der Begriff "Topologischer Isolator" (TI) beschreibt eine neuartige Klasse von Verbindungen deren Inneres (engl. Bulk) isolierend ist, dieses Innere aber gleichzeitig und zwangsläufig eine metallisch leitende Oberfläche aufweist. Dies ist begründet in der nicht-trivialen Topologie dieser Materialien, welche durch eine spezielle Invertierung einzelner Bänder in der Bandstruktur und der Spin-Bahn-Kopplung im Materialinneren hervorgerufen ist. Diese topologisch geschützten, metallischen Oberflächenzustände sind gekennzeichnet durch eine masselose Dirac Dispersionsrelation und gekoppelte Helizität der Spinpolarisation, welche die Rückstreuung der Ladungsträger verbietet und somit zur Stabilisierung der Zustände gegenüber Störungen beiträgt. Auf Grundlage dieser außergewöhnlichen Merkmale haben Theoretiker eine Fülle neuer Phänomene und spannender Experimente vorhergesagt. Deren experimentelle Überprüfung steht jedoch noch aus, geschweige denn deren Umsetzung in Anwendungen, wie zum Beispiel die Erzeugung von Majorana Teilchen, fortgeschrittene Spintronik, oder die Realisierung von Quantencomputern. Aufgrund ihrer relativ einfachen Bandstruktur, welche nur einen Dirac-Kegel an der Oberfläche aufweist, haben die 3D TI Bi2Te3 und Bi2Se3 in den letzten Jahren großes Interesse erlangt. Weiterhin besitzen diese Materialien eine merkliche Bandlücke von bis zu ~0,3 eV, welche sogar Anwendungen bei Raumtemperatur ermöglichen könnten. Dennoch ist deren experimentelle Umsetzung nachwievor eine enorme Herausforderung. Das Haupthindernis, welches bis jetzt insbesondere die elektrische Charakterisierung the topologischen Oberflächenzustände behindert hat, ist die zusätzliche Leitfähigkeit des Materialinneren, welche durch Kristalldefekte und Beimischungen, sowie die Verunreinigung der Probenoberfläche durch Luftexposition bedingt wird. Die vorliegende Arbeit liefert einen Beitrag zu aktuellen den Anstrengungen in der Verbesserung der Probenqualität der TI um die Leitfähigkeit des Materialinneren zu unterdrücken, sowie die anschließende Untersuchung der elektrischen Eigenschaften unter kontrollierten Bedingungen durchzuführen. Weiterhin sollen geeignete Deckschichten identifiziert werden, welche die besonderen elektronischen Merkmale der TI nicht beeinflussen sowie diese gegen äußere Einflüsse schützen, und somit die Durchführung anspruchsvoller ex situ Experimente ermöglichen können. Die untersuchten Bi2Te3 Schichten wurden mittels Molekularstrahlepitaxie (MBE) hergestellt. Es konnte gezeigt werden, dass es allein durch Optimierung der Wachstumsbedingungen möglich ist Proben herzustellen, die gleichbleibend isolierende Eigenschaften des TI Inneren aufweisen und Eindomänen-Ausrichtung besitzen. Die zentralen Faktoren sind hierbei die Aufrechterhaltung eines Flussratenverhältnisses von Te/Bi ~8 der einzelnen Elemente, sowie die Wahl einer ausreichend hohen Substrattemperatur, um ein vollständiges Abdampfen (Destillation) des überschüssigen Tellur zu erreichen. Weiterhin müssen Substrate mit gut angepassten Gitterparametern verwendet werden, welches bei BaF2 (111) gegeben ist. Optimales MBE Wachstum konnte durch ein Zwei-Stufen Prozess bei Substrattemperaturen von 220°C und 250°C und einer Bi-Verdampfungsrate von 1 Å/min erreicht werden. Die nachfolgende Charakterisierung der strukturellen Eigenschaften, Photoelektronenspektroskopie, sowie temperaturabhängige Leitfähigkeitsmessungen wurden alle in einem zusammenhängenden Ultrahochvakuum-System durchgeführt. Auf diese Weise wird eine zuverlässige Erfassung der intrinsischen Eigenschaften der TI sichergestellt. Zur Überprüfung, ob die Leitfähigkeit der Proben tatsächlich nur durch die Oberflächenzustände hervorgerufen wird, wurden Filme mit Schichtdicken im Bereich von 10 bis 50 Quintupel-Lagen (QL; 1QL~ 1 nm) hergestellt und charakterisiert. Winkelaufgelöste Photoelektronenspektroskopie (ARPES) belegt, dass das chemische Potential (Fermi-Niveau) in allen Proben innerhalb der Bandlücke der Bandstruktur des Materialinneren liegt und nur von den topologisch geschützten Oberflächenzuständen gekreuzt wird, welche die charakteristische lineare Dirac Dispersionsrelation aufweisen. Die temperaturabhängigen Widerstandsmessungen zeigen ein metallisches Verhalten aller Proben. Bei der Variation der Schichtdicke von 10 zu 50QL wird eine Streuung des Flächenwiderstandes vom Faktor 1,3 bei 14K und 1,5 bei Raumtemperatur beobachtet. Dies beweist, dass die gemessene Leitfähigkeit vorrangig durch die topologisch geschützten Oberflächenzustände hervorgerufen wird. Eine geringe Oberflächenladungsträgerkonzentration im Bereich von 2–4*10^12 cm^−2 und hohe Mobilitätswerte von bis zu 4600 cm2/Vs wurden erreicht. Weiterhin wurden die negativen Auswirkungen auf die Eigenschaften der TI durch Luftexposition quantifiziert, welches die Notwendigkeit belegt, die Oberfläche der TI vor Umgebungseinflüssen zu schützen. Die Proben verhalten sich inert gegenüber reinem Sauerstoff, daher ist Wasser aus der Luftfeuchte höchstwahrscheinlich der Hauptgrund für die beobachtbare Verschlechterung. Darüber hinaus konnte epitaktisch gewachsenes Tellur als geeignete Deckschicht ausfindig gemacht werden, welches die Eigenschaften der Bi2Te3 Filme nicht beeinflusst, sowie gegen Veränderungen durch Luftexposition schützt. Die gewonnenen Erkenntnisse stellen eine ideale Grundlage für weiterführende Untersuchungen dar und ebnen den Weg zur Entwicklung von Bauelementen welche die spezifischen Besonderheiten der topologischen Oberflächenzustände

Die vorliegende Arbeit befasst sich mit der Untersuchung der mikromagnetischen Domänenstruktur und des gekoppelten magneto-strukturellen Phasenübergangs dünner epitaktischer MnAs-Filme auf GaAs. Im Besonderen wird der Einfluss der Substratorientierung, der Filmdicke und eines externen magnetischen Feldes auf die magnetischen und strukturellen Eigenschaften untersucht. Dabei kommen die komplementären Untersuchungsmethoden AFM (atomic force microscopy) / MFM (magnetic force microscopy) und LEEM (low energy electron microscopy) / XMCDPEEM (X-ray magnetic circular dichroism photoemission electron microscopy) zum Einsatz. Im Zuge des Phasenübergangs erster Ordnung zeigen MnAs Filme auf GaAs (001) und (311)A eine regelmäßige Anordnung ferromagnetischer alpha-MnAs und paramagnetischer beta-MnAs Streifen. Die Breite der Streifen ist eine Funktion der Temperatur, während die Periodizität eine lineare Funktion der Filmdicke ist. Die Domänenstruktur hängt stark von der Breite bzw. dem Abstand der ferromagnetischen Streifen ab, da diese direkt die Formanisotropie bzw. die magnetische Kopplung beeinflussen. Die Domänenstrukturen wird, abhängig von der Zahl der Subdomänen entlang der leichten Magnetisierungsrichtung, klassifiziert, wobei bis zu drei elementare Domänentypen beobachtet werden. Bei MnAs-Filmen die auf der GaAs (111)B Oberfläche gewachsen wurden, führt die Epitaxie zu einem geänderten Spannungszustands des Films, wobei eine erhöhte Phasenübergangstemperatur beobachtet wird. Durch temperaturabhängige XMCDPEEM-, AFM- und MFM-Messungen kann gezeigt werden, daß durch den lokalen Abbau der Verspannung in der Nähe eines Risses die Phasenübergangstemperatur lokal erhöht ist. Um Ummagnetisierungsprozesse auf einer mikroskopischen Skala untersuchen zu können und um den Einfluß eines magnetischen Feldes auf die Domänenstruktur sichtbar zu machen, wurde das temperaturvariable Rastersondenmikroskop um einen variablen Magnetfeldaufbau ergänzt.
This work presents the study of the micromagnetic domain structure and the coupled magneto-structural phase transition of epitaxial MnAs thin films on GaAs. In particular, the influence of substrate orientation, film thickness and external magnetic field on the magnetic and structural properties are investigated, employing the complementary measurement techniques atomic force microscopy (AFM) / magnetic force microscopy (MFM) and low energy electron microscopy (LEEM) / X-ray magnetic circular dichroism photoemission electron microscopy (XMCDPEEM. In the course of the first-order phase transition MnAs films on GaAs (001) and (311)A substrates show a regular array of ferromagnetic alpha- and paramagnetic beta-MnAs stripes. The width of the ferromagnetic stripes are a function of the temperature, whereas the periodicity of the stripe pattern is a function of the film thickness. The domain structure strongly depends on the width and the distance of the ferromagnetic stripes, as it directly affects the shape anisotropy and magnetic coupling, respectively. The domain patterns are classified depending on the number of subdomains along the easy axis direction. Up to three basic domain types can be distinguished. For MnAs films grown on GaAs (111)B, the epitaxy leads to a different strain state of the film, resulting in polygonal ferromagnetic structures embedded in a honeycomb-like paramagnetic network, and a higher phase transition temperature. Using temperature-dependent AFM, MFM and XMCDPEEM it is shown that the local strain relaxation in the vicinity of cracks in the MnAs film results in a locally increased phase transition temperature. In order to study magnetization reversal processes on a microscopic scale, as well as the influence of the magnetic field on the domain structure, a variable-magnetic field set-up is employed.

Le siliciure ysi 1,7 en phase hexagonale s'epitaxie sur un support si(iii) avec un accord de maille quasi parfait. La couche resultante (100-200 a) contient des grains de 50 a 100 a. Une forte anisotropie de structure de bandes est revelee par photoemission. Les lacunes si s'ordonnent en trois dimensions. Mesures electriques: comportement de type semi-metallique et faible hauteur de barriere de schottky pour le contact ysi 1,7/si dope n. Si peut se reepitaxier sur une couche de siliciure d'y elle-meme epitaxiee sur un support si(iii). Un echantillon est obtenu, avec toutefois des defauts d'empilement dans la couche si. La fabrication de couches multiples de cette nature est envisageable

Nous avons surtout utilise le mode "haute resolution" sur un microscope a 200kv. La resolution obtenue etait de 2. 4 a. La technique de peparation d'echantillons que nous avons mise au point pour l'etude du procede de passivation (si::(3)n::(4)/gaas) nous a permis de caracteriser dans de tres bonnes conditions les multicouches pour rayons x mous et les heterostructures de croissance epitaxiale. Ce travail fut un suivi des procedes en conjugaison avec d'autres techniques de caracterisation. La comparaison des resultats de ces diverses techniques nous a permis d'apprehender la chimie et la physique des interfaces dans les materiaux etudies

Rare-earth nickelates (RENiO3), a family of transition metal oxides, exhibit a complex phase diagram involving electronic, magnetic, and structural phase transitions. While LaNiO3 remains paramagnetic, metallic down to very low temperature, RENiO3 members with RE=Nd, Pr exhibit simultaneous metal-insulator transition (MIT), paramagnetic to antiferromagnetic transition, structural phase transition and a bond disproportionation (BD) transition as a function of temperature. The other members of the series such as EuNiO3, SmNiO3, etc. first undergo simultaneous MIT, BD, and structural phase transition and further becomes antiferromagnetic upon lowering the temperature. Understanding the origin of the MIT in this family remains a challenging problem and has attracted a lot of attention in recent times. The MIT temperature can be tuned by a variety of parameters such as chemical doping, pressure, epitaxial strain, light, etc. In this thesis, we have grown epitaxial thin films of doped rare-earth nickelates and investigated their electronic and magnetic behavior using several experimental techniques, including synchrotron-based measurements. In the first part, we have investigated Ca2+ (divalent) and Ce4+ (tetravalent) doped NdNiO3 thin films. Doping with divalent ions at the Nd sites introduces holes, whereas doping with tetravalent ions introduces electrons, resulting in a change in the formal valence of Ni. Both electron and hole doping suppress the insulating phases with asymmetric suppression rates for the metal-insulator phase transition. We have shown that the effective charge transfer energy changes with carrier doping and the formation of the BD phase is not favored above a critical doping, suppressing the insulating phase. Our research clearly shows that the appearance of BD mode is critical for the appearance of MIT in RENiO3 family. In the second part, we have investigated rare-earth nickelate in high entropy oxide (HEO) form. HEOs are defined as a class of materials containing equimolar or nearly equimolar portions of five or more elements stabilizing in a single phase. HEOs have been explored in recent years to achieve tunable properties in unexplored parts of the complex phase diagram. However, epitaxial stabilization of such multi-element systems is challenging, and it is unknown how epitaxial strain will affect the electronic and magnetic behavior of HEO. We have been able to grow (LaPrNdSmEu)0.2NiO3 [(LPNSE)NO] thin films on different substrates having different epitaxial strains. We have shown that, in spite of having multi-element and strong disorder at the RE site, the average tolerance factor determines the electronic and magnetic properties. We further studied the strain effect on MIT of those HEO thin films. We have observed that (LPNSE)NO film grown under tensile strain (substrates: NdGaO3 and SrTiO3) exhibits a metal-insulator transition. We have found that this transition can be completely suppressed by compressive strain exerted by SrLaAlO4 substrate. Surprisingly, HEO film, grown on SrPrGaO4 substrate, where the strain is almost negligible, does not exhibit any MIT. We have further demonstrated that the octahedral rotation pattern of the substrate governs the octahedral rotation and Ni-O-Ni bond angle of the epitaxial thin films, which in turn controls the MIT. In the third part, we have explored (LPNSE)NO thin films as electrocatalysts. Oxygen evolution reaction (OER) is a key process in several alternative energy generation platforms such as solar and electric driven water splitting, fuel cells, rechargeable metal-air batteries, etc. We have investigated the thickness dependent OER of (LPNSE)NO thin films and found that the increase of film thickness results in higher OER activity. X-ray absorption spectroscopy measurements find an increase in Ni d-O p covalency and a decrease in charge transfer energy with the increase in film thickness. These facilitate higher charge transfer between Ni and surface adsorbates, resulting in higher OER activity.

A precise understanding of the fundamental correlation between synthesis, microstructure and physical properties is of vital importance towards rational design of improved functional epitaxial thin films. With the presence of heterogeneous interface and associated inhomogeneous lattice strain, film microstructure becomes sensitive to subtle interfacial perturbations and hence may exhibit intriguing physical properties. Control of the epitaxial film functionality requires accurate knowledge of the actual film chemistry, interfacial defects and associated strain field. This dissertation reports in-depth microstructural characterization of the intrinsic chemical inhomogeneity in selected epitaxial thin films including superconducting Fe1+yTe1-xSex/SrTiO3(STO) heterogeneous systems, the flux-pinning defects at both of conversional YBa2Cu3O7-δ (YBCO)/substrate lateral interfaces and vertical interfaces of YBCO&BaSnO3(BSO) nanocomposite films, and the misfit dislocation core configurations of STO/MgO and MgO/STO heterostructures pair, using the state-of-the-art aberration-corrected scanning transmission electron microscopy (CS-corrected STEM) in combination with geometric phase analysis (GPA). For the first time, the local atomic arrangement of Te and Se as well as interstitial Fe(2) has been clearly revealed in superconducting Fe1+yTe1-xSex/STO epitaxial films. We found that the film growth atmosphere can greatly affect the film stoichiometry, the homogeneity of Se/Te ordering and thus the overall film superconductivity. YBCO/substrate interface mismatch and YBCO&BSO vertical interface contact have been explored through substrate selection and doping-concentration variation. We observed a diverse nature of intrinsic defects in different YBCO/substrate heterosystems; thermal stable defects capable of maintaining individual strain field have been found effective in flux-pinning. Along the vertical heterointerface of YBCO/BSO, misfit dislocations were found throughout the film thickness. It adds another dimension to the flux-pinning landscape design. Four basic misfit dislocation core configurations of a STO/MgO heterosystem have been identified, and found strongly dependent on the actual interface disordering such as substrate atomic-height steps and interdiffussion. To precisely quantify the heterointerface lattice strain, we first conducted systematic investigations on the accuracy of STEM-based GPA. Follow our protocol, 1 pm accuracy has been proven in the STEM fast-scan direction with a spatial resolution less than 1 nm. The effectiveness and reliability of this optimized GPA strain profile were demonstrated in both applications of a relaxed STO/MgO and a partially strained LaAlO3/STO heterointerfaces, respectively.

The multiferroics are an important class of multifunctional material which simultaneously possess spontaneous ferroelectric polarization and magnetic ordering. If there exists a coupling between the ferroelectricity and the magnetic ordering, the materials are known as magneto electric (ME) multiferroic materials. The coupling between the magnetic and electric order parameters allows to tune the magnetic properties by an electric field and vice versa. Multiferroic materials are promising candidate for designing new spintronic devices, advanced sensors, high density ferroelectric memory devices and the emerging category of four-state memory devices. In multiferroic memory devices, data can be written electrically using its ferroelectric property and can be read magnetically without causing any Joule heating. Depending on the origin of ferroelectricity and magnetic orderings, multiferroics can be divided into two categories: type I and type II multiferroics. The type I multiferroics have different sources of ferroelectricity and magnetism. On the other hand, ferroelectricity is induced by the magnetic ordering in type II multiferroic materials and they have a strong ME coupling. However, even after extensive investigations into different families of compounds, a multiferroic material with high-enough polarization and magnetization suitable for practical applications has not been realized yet. In order to overcome this problem, composite multiferroics are designed by combining a ferroelectric and a ferromagnetic material. Recently composite multiferroics have drawn significant attention due to its enormous design flexibility which can be used for a wide range of applications. In this thesis, a thorough study of the structural, electrical, and magnetic properties of multiferroic BiFeO3 and CuO epitaxial thin films is carried out. BiFeO3 is a type I multiferroic material with a perovskite distorted rhombohedral (R3c) crystal structure. It is ferroelectric (TC = 1123 K) and G-type antiferromagnetic (TN = 643 K) at room temperature. Antiferromagnetism in BiFeO3 arises from the Fe sublattice having d5 configuration whereas ferroelectricity appears due to the directional orientation of 6s lone pair electrons of the Bi3+ ion. We observed that the crystal structure of BiFeO3 thin film gets altered depending on lattice misfit stress caused by the substrate which in turn modifies its magnetic properties through strong magneto-structural coupling. Furthermore, a signature of magneto-(di)electric coupling and exchange bias effect were observed between the BiFeO3 and SrRuO3 layers of a heterostructure. On the other hand, CuO is a type II multiferroic material where ferroelectricity is generated between 213 K and 230 K due to incommensurate spiral magnetic spin ordering along its crystallographic ‘b’ axis. We found that CuO thin films can be grown in the direction of its static polarization axis by proper choice of substrate and the temperature dependent magnetic properties of CuO thin films vary depending on its crystallographic orientations due to strong magneto-structural coupling. Chapter 1 provides a general introduction to various physical phenomena, such as ferroelectricity, ferromagnetism, antiferromagnetism, multiferroicity, magneto-electric coupling, and different magnetic interactions, like Dzyaloshinskii-Moriya interaction, and exchange bias effect. Basic concepts of impedance spectroscopy, dielectrics and perovskite structures are also discussed. General introductions of different materials, which are studied in the thesis, and the motivation of choosing them are incorporated at the end of the chapter. Chapter 2 contains the description of thin film growth technique and different steps of device fabrication process. Different characterization techniques, the instruments used for the characterizations and the working-principle of those instruments have been summarized in the chapter. Chapter 3 focuses on the variation of magnetic properties and crystal structure with the thickness of BiFeO3 thin films. BiFeO3 thin films of different thicknesses, ranging from 16 nm to 60 nm, were grown on (001) SrTiO3 substrate by PLD technique. Detailed x-ray diffraction studies show that the 16 nm, 20 nm and 30 nm films have “R-like” crystallographic phase with an out-of-plane lattice parameter of 4.06 Å whereas the 45 nm and 60 nm films have “R-like” and ‘T-like” crystallographic phases simultaneously. The “T-like” phase has an out-of-plane lattice parameter of 4.65 Å and a c/a ratio of 1.25, resembling a tetragonal crystal structure. Off-specular reciprocal space mapping and azimuthal φ scan show that the “T-like” phase deviates from an ideal tetragonal crystal structure by a monoclinic tilt. The occurrence of the “T-like” phase is associated with the formation of a very thin layer of parasitic Bi2O3 phase which appears in between two film-thicknesses of 30 nm and 45 nm and BiFeO3 grows in “T-like” phase thereafter. High lattice mismatch between Bi2O3 phase and BiFeO3 phase causes more distorted unit cell in “T-like” phase with a high c/a ration. Parasitic Bi2O3 phase appears because of slightly higher partial oxygen pressure used during the growth which prevents the formation of the parasitic ferrimagnetic γFe2O3 phase in the films. Moreover, our XPS studies confirmed that the films contain Fe3+ only without any trace of Fe2+ within a resolution of few atomic percentages and the magnetic signals measured in our experiments are entirely from the BiFeO3 phase. The saturation magnetizations of the films were found to increase with decreasing thickness. At room temperature, the saturation magnetization of a 16 nm-thick BiFeO3 thin film is 87 emu/cc but it goes down to 9 emu/cc when the thickness increases to 60 nm. Moreover, it was observed that the 16 nm thick film is magnetically more anisotropic in comparison to the 60 nm thick film and there is an apparent out-of-plane magnetic hard axis in the 16 nm film. Summarizing the results obtained from the films with different thicknesses, it can be concluded that the vanishing magnetic anisotropy is related to the structural transformation of the film. Chapter 4 provides a detailed study of the variation of magnetic properties of a BiFeO3 thin film with its crystal structure. BiFeO3 thin films of different thicknesses were grown on orthorhombic (001) NdGaO3 substrate. In-depth x-ray diffraction studies and off-specular reciprocal space mapping show that a 15 nm thick BiFeO3 film grows with monoclinic crystal symmetry (Cm) with an out-of-plane lattice parameter of 4.187 Å on the NdGaO3 substrate. The crystal structure was further verified by the TEM studies which showed a good agreement with the results obtained from x-ray diffraction studies. To probe the ferroelectric nature of the monoclinic BiFeO3 film, piezo response force microscopy was performed. It was found that the oppositely oriented ferroelectric domains have 180° phase contrast and a phase vs. voltage hysteresis loop gets generated when the domains are switched between two antiparallel directions. DC magnetic measurements at room temperature showed that the saturation magnetization of the 15 nm film with Cm crystal symmetry is as high as ~250 emu/cc. Experimental evidence confirmed that the films are free from all magnetic parasitic phases and the high saturation magnetization comes solely from the BiFeO3 phase. For comparative study, BiFeO3 films of similar thickness were deposited on (001) SrTiO3 under identical conditions which grew in “R-like” crystal structures. We saw that “R-like” BiFeO3 films have saturation magnetization 2.5 times lower (~100 emu/cc) than that of the film with Cm structure grown on NdGaO3. Our observation was further supported by density functional theory calculations which show that BiFeO3 has a ferromagnetic ground state in the Cm crystal phase. The theoretically obtained magnetic moment is 266 emu/cc which is very close to magnetization values found experimentally. Chapter 5 deals with the magnetic interaction and the magneto-electric coupling between the BiFeO3 and SrRuO3 layers of a heterostructure. BiFeO3/SrRuO3 heterostructures were grown on (001) SrTiO3 substrate by PLD technique. The ferroelectric nature of the top BiFeO3 layer was probed by out-of-plane piezo response force microscopy technique. Temperature dependent magnetization measurements of the heterostructure show a sharp ferromagnetic to paramagnetic transition at 160 K which arises from the bottom SrRuO3 layer. Therefore, the heterostructure is ferroelectric and ferromagnetic below 160 K. Magnetic interactions between the two layers were investigated by isothermal magnetic hysteresis loop (M-H) measurement in a SQUID magnetometer. The M-H measurements at 10 K showed a two-step magnetic hysteresis loop which implies that magnetic moments of the SrRuO3 layer get pinned by the magnetic interaction between the two layers. During magnetization reversal process, the pinned magnetic moments switch at a higher magnetic field and generate the second step of the hysteresis loop whereas the first step appears at a lower magnetic field during the switching of the free SrRuO3 moments. The amount of the pinned SrRuO3 moments depends on the thickness of the BiFeO3 layer as the magnetic properties of a BiFeO3 thin film are related to its thickness. Moreover, evidence of the exchange bias effect was also found in the heterostructure. Field-cooled M-H measurement shows that the second step of the hysteresis loop shifts in two opposite directions along the magnetic field axis depending on the polarity of the cooling field whereas the first step doesn’t respond to the cooling field. This confirms that the exchange bias effect is directly related to the pinned magnetic moments of the SrRuO3 layer. The total amount of pinned moment and hence the exchange bias effect reduces with increasing temperature and disappears completely above 100 K. A strong coupling between the electrical properties of the BiFeO3 layer and the magnetic properties of the SrRuO3 layer was also observed in the heterostructure. To carry out electrical measurements, interdigitated gold electrodes were fabricated on the BiFeO3 layer of the heterostructure by standard photolithography, magnetron sputtering, and lift-off procedure. Temperature dependent resistance and reactance measurements of the heterostructure at different frequencies show anomalies at ferromagnetic TC of the bottom SrRuO3 layer. Moreover, temperature dependent capacitance measurement at 0 T and at 5 T magnetic fields also showed anomalies near 160 K which indicate that the electrical properties of the heterostructure are affected by the magnetic transition of the SrRuO3 layer. Furthermore, impedance spectroscopy measurements were carried out at different constant temperatures and the corresponding Nyquist plots were fitted with an equivalent circuit model. Remarkably, the capacitance and resistance of the equivalent circuit corresponding to the BiFeO3 layer of the heterostructure, show anomalies at 160 K. Absence of any dielectric anomaly at 160 K in pure BiFeO3 confirms that the observed ones appear because of the magnetic phase transition of the bottom SrRuO3 layer. Therefore, the BiFeO3/SrRuO3 heterostructure has ferroelectric and ferromagnetic properties along with a strong magneto-electric coupling between the layers which can be a promising candidate for the composite multiferroic. Chapter 6 describes a correlation between the crystal structure and magnetic properties of CuO thin film. CuO thin films were grown on (001) SrTiO3, (110) SrTiO3, and (111) Si substrate with MgO buffer layers by PLD technique. On (110) SrTiO3 substrate, CuO thin films grow along [010] direction, which is the direction of ferroelectric polarization of CuO, but growth direction becomes [111] when (001) SrTiO3 substrate is used. The CuO film becomes polycrystalline when it is grown on (111) Si substrate. To find the in-plane epitaxial relations between the substrate and the two layers, cross-sectional TEM of the heterostructure grown on (110) SrTiO3 was carried out. HRTEM images showed very sharp interfaces between the layers indicating high-quality growth of the heterostructure. The epitaxial relations were deduced from the SAED pattern and the FFT pattern of the HRTEM images. Distinctly different temperature dependent magnetic properties were found for three differently oriented CuO films. Two anomalies at 213 K and 230 K are clearly visible in temperature dependent magnetization (M vs. T) plot of the heterostructure with (010) CuO film which are associated with the two magnetic transitions of CuO. On the other hand, no such anomaly was observed in M vs. T plot of the heterostructure with (111) CuO film. The heterostructure with polycrystalline CuO film shows a very weak magnetic anomaly at 230 K in its M vs. T plot. It can be concluded from our studies that the contrasting magnetic behaviours of these three heterostructures are due to the difference in epitaxial orientations of the CuO layers. Moreover, CuO thin films can be successfully grown in the direction of static ferroelectric polarization which is the ‘b’ axis of its monoclinic crystal structure. Chapter 7 concludes with general findings pertaining to various observations made in the different chapters. Prospects for future work are briefly outlined in this chapter.

This chapter reviews some of the initial developments and recently introduced potential application concepts related to domain walls in ferroelectrics and multiferroics. It gives a special (non-exclusive) focus on the heavily investigated bismuth ferrite BiFeO3 system as one of the rare examples of a single phase room-temperature multiferroic system that can be widely tailored in application relevant epitaxial thin films. Here, DWs as well as other topological structures reveal new ways to novel tailored states of matter with a wide range of electronic properties. Domain wall electronics, particularly with ferroelectrics and multiferroics, provides new nanotechnological concepts for identifying, understanding, and designing new material properties. However, this chapter observes that there has been very little work done on controlling electronic correlations.

The ultra-short pulse laser has the potential in selective nano-structuring of thin-films layers by adjusting the wavelength of laser radiation depending on optical properties of the thin- film and the substrate that will solve its efficiency and stability issues in a one-step process, which is a promising methodology for thin-film solar cell fabrication that are fabricated through a sequence of vapor deposition and scribing processes. The review is performed to further understand the structure of the laser modified surface and the nature of dopants and defects in the crystalline grains. Using low temperature studies, the electronic levels of the dopant and its configuration with the lattice could be probed. The review is also explored the concept of using thin films of silicon as the laser irradiation substrate and for enhanced the visible and infrared absorption of films of silicon with thicknesses of few micrometer. Although the review is made good progress studying the properties of new material and incorporation into device but there are many unanswered questions and exciting avenues of research are also explored with femtosecond laser irradiated silicon.

A large number of studies have been done for multiple quantum wells of inorganic semiconductor, because confined electronic system in a thin layer shows various properties essential for optoelectronic application. Since the quantum size effect increases when the electron system approaches lower dimension, fabrication of higher order confined structures has been attempted to advance the properties. On the other hand, there are many substances having one-dimensional structure in the nature. It can be thought that the one-dimensionality is prospective to realize a higher order confined system.

Cyanine dyes can be used for formation of composite molecular Langmuir- Blodgett (LB) films with interesting photosensitive properties and prospects of non-linear optical applications1. For water-soluble cyanine dye molecules the possibility of epitaxial single crystal growth by adsorption of dye molecules onto the surface of oppositely charged lipid monolayers was demonstrated recently2. X-ray analysis, electron diffraction and fluorescence microscopy give information about unit cell parameters, average thickness of these composite LB films, and their morphology on a micron scale2,3 Data about surface morphology and roughness on a submicron scale and details of the molecular packing in these films have not yet been obtained. Here we discuss the results of atomic force microscopy (AFM) and quantitative X-ray reflectivity studies of the molecular structures of these composite molecular films.

Studies on molecular beam epitaxial growth for organic molecules (OMBE) have been attracting attentions as a new fabrication method for preparation of ultra-thin films of organic molecules. Molecular beam epitaxy (MBE) has been developed as a deposition technique for the controlled growth of atomically flat multilayer structures. Recently, this method has been applied to organic materials, while the mechanism of the growth has not been thoroughly understood compared to that of semiconductor. In this report, the growth mechanism of Lead Phthalocyanine (PbPc) thin films onto highly oriented pyrolytic graphite (HOPG) substrate by OMBE method was investigated using in situ refraction high energy electron diffraction (RHEED). RHEED has been widely used for in situ monitoring of the thin film MBE growth. The intensity of the RHEED specular beam spot oscillates according to the layer-by-layer growth[1]. In 1981, Joyce et.al. observed an oscillated phenomena of RHEED intensity with growth of semiconductor for the first time[2]. After that, a lot of work in the semiconductor field have been reported and developed. Although application for organic materials is expected, so far RHEED oscillation studies have only been reported for Copper Phthalocyanine (CuPc) on MoS2 substrate[3]. Therefore, we explored the possibilities of using in situ RHEED observations to investigate the OMBE growth mechanism of PbPc on HOPG system.

Several desirable characteristics make single crystal beryllium films and Be-Ge heterostructures of interest in various areas. Because of its very low absorption at soft x-ray wavelengths,1 Be is attractive to the x-ray optics community for use as a spacer material in multilayer mirrors. Such mirrors would be of use in x-ray microscopy,2,3 lithography,4,5 and astronomy.6 Be is also potentially useful in infrared (IR) optics because of its high IR reflectance and good thermal properties.7 In addition to these optical properties, the electrical and electronic properties of Be thin films could also be utilized in other areas, such as electromagnetic shielding in space applications,8 studies of superconductivity,9,10,11,12,13,14,15 and hot-electron transistor technology.16

Modern epitaxial crystal growth techniques have made it possible to tailor compositionally modulated thin films on an atomic level. However, further progress in control of epitaxial growth is limited by a relative lack of useful in situ techniques for surface analysis during growth. The most widely used in situ structural analysis technique for molecular beam epitaxial (MBE) growth is reflection high energy electron diffraction (RHEED). Its long working distance and high data rate have prompted investigations of RHEED oscillations and other RHEED dynamic features by many groups. However, there is currently no widely employed in situ chemical analysis technique that has the advantages of RHEED[1]. Hence, for example, the ability to perform real-time control of alloy composition in epitaxial films is limited.

Nanostructures are defined to be ultrasmall structures and devices with dimensions less than or equal to 100 nm. Conventional methods for making thin film structures involve exposure of a thin layer of a polymer resist on a suitable substrate to define a pattern, which is than developed and used to fabricate the structures either by deposition, or by etching [1]. The feature sizes of the structures depend both on the resolution of the resist and the imaging method used to expose the resist [1,2]. The materials quality of nanostrustures is also an important factor. In many cases the nanostructures must be crystallographically perfect. Resistless methods of patterning, followed by epitaxial growth could significantly simplify nanofabrication by eliminating a number of processing steps associated with the application, exposure, development, and removal of the resist. The advantage of epitaxial growth is that unlike etching, it is less prone to produce damage in the regions adjacent to the structures. The molecular size effect with polymer based resists such as PMMA is believed to be a significant factor in limiting the resolution (“grain size”) in electron beam lithography (EBL) to 10 nm. Surface adsorption layers such as the hydride layer on the Si surface are characterized by relatively strong chemical bonding which produces a highly uniform coverage that terminates at a single monolayer. Because of these properties surface adsorption layers are attractive candidates as ultrathin, ultrahigh resolution resists for electron beam patterning.

The manipulation of strain in micromachined silicon structures is an important aspect of the design of emerging mechanical and electronic devices. Strain also has a fundamental role in the formation of devices through its effects on surface processes in epitaxial growth including diffusion and can be an important tool for studying these processes. Microfabricated silicon structures offer the opportunity to control the strain at length scales of less than one micron to several hundred microns. Synchrotron x-ray microdiffraction allows simultaneous independent measurements of the strain and bending in these structures. Microdiffraction measurements show that bending is the dominant source of strain in a prototypical microfabricated Si bridge loaded at its ends by silicon nitride thin films. The total strain difference between the top and bottom of the bent bridge exceeds 0.1% in our prototype structures and can potentially be increased in optimized devices.